Use of a catalyst comprising a beta silicon carbide support in a selective hydrodesulphurization process

ABSTRACT

The invention concerns the use of supported catalysts comprising at least one metal or metallic compound of a metal from group VI and/or group VIII deposited on a support essentially constituted by β silicon carbide in a process for selective hydrodesulphurization of an olefinic hydrocarbon feed that is substantially free of polynuclear aromatics and metals. The invention can be used to carry out deep desulphurization of catalytically cracked gasoline cuts with very limited saturation of olefins and thus a minimum loss of octane number.

FIELD OF THE INVENTION

The present invention relates to the oil refining industry, more particularly to the production of gasoline bases from different units for converting oil cuts, in particular cracking units.

Thermal cracking units, for example for visbreaking or cokefaction, or catalytic cracking, for example for fluidized bed catalytic cracking, produce unsaturated gasoline cuts comprising large quantities of aromatics and olefins. Such gasoline cuts generally have substantial levels of sulphur, for example in the range 200 to 3000 ppm by weight, which is incompatible with general specifications for gasoline fuel. Thus, such cuts have to be deeply desulphurized to reduce the sulphur content to less than 30 ppm by weight or even 10 ppm by weight in accordance with current specifications, or to satisfy future specifications. Industrial processes that are in current use to carry out said operation are catalytic hydrodesulphurization processes.

Said cuts typically have an end point of 260° C. or less. Thus, they are substantially free of heavy aromatics. Said fractions are also substantially free of metals such as nickel, vanadium or mercury which may poison the catalyst and/or prevent its regeneration. Deep hydrodesulphurization of said fractions, however, is very difficult because of a specific technical problem: those fractions have to be efficiently desulphurized without, however, substantially hydrogenating the olefins that are present. A reduction in the olefin content and thus a correlative increase in the paraffin content would result in an unacceptable drop in the octane number of the gasoline fuel, an essential parameter for selling price. Thus, a method for selective hydrodesulphurization of the treated fractions is sought, i.e. hydrodesulphurization which can eliminate sulphur with minimum hydrogenation of the olefins in the feed.

PRIOR ART

Catalysts used industrially for that operation are typically based on alumina, typically comprising at least one metal or metallic compound of a metal from group VIII of the elements periodic table (group including nickel, iron and cobalt, by which the version of the elements periodic table which is used can be identified) and/or at least one metal from group VIB of that table (group comprising molybdenum and tungsten). A variety of techniques have already been employed to increase the hydrodesulphurization selectivity: modifying the operating conditions in which desulphurization is carried out and/or modifying the hydrodesulphurization catalyst to improve selectivity. Several methods have been proposed to improve the selectivity of the catalyst. Non exhaustive examples which can be cited are:

-   -   U.S. Pat. No. 4,140,626, concerning a process for hydrotreating         cracked naphtha using a catalyst containing a metal from group         VIB and from group VIII deposited on a support composed of at         least 70% by weight magnesium oxide;     -   U.S. Pat. No. 3,957,625, for a process for selective         desulphurization of cracked gasoline using a         cobalt-molybdenum/alumina type catalyst with a promoter selected         from barium, magnesium, cadmium and rare earths;     -   U.S. Pat. No. 5,348,928, concerning a method for preparing and         formulating a selective hydrotreatment catalyst comprising a         metal from group VIB containing 4% to 20% oxide equivalent by         weight and a metal from group VIII containing 0.5% to 10% oxide         equivalent by weight. The support contains 0.5% to 50% oxide         equivalent by weight of magnesium and 0.02% to 10% oxide         equivalent of an alkali with respect to the total catalyst mass.

However, known prior art catalysts consume quantities of olefins which remain substantial and prejudicial to selling price of the fuel. This consumption depends on the treated feed, the hydrotreatment conditions, but also on the required degree of desulphurization. One aim of the invention is to use particular catalysts that can improve the hydrodesulphurization selectivity for light cuts comprising olefins. A further aim of the invention is to propose a process for selective hydrodesulphurization employing said catalysts.

DESCRIPTION OF THE INVENTION

Catalysts comprising a support constituted by β silicon carbide are already known: European patent EP-B1-0 313 480 describes a catalyst for hydrotreating oil distillation cuts, said catalyst comprising a support constituted by β silicon carbide (SiC).

That support is described as advantageous in that it is resistant to poisoning by both coke and metals. Coke accumulation is described as being linked to the presence of high molecular weight aromatics, and the presence of metals (principally nickel and vanadium) is described as the result of the presence of said metals in heavy oil cuts. The desulphurizing activity is measured by comparison with other catalysts (comprising a catalyst with an alumina support with a specific surface area of 220 m²/g) in the case of thiophene. Such a comparison shows that the desulphurizing activity (moles of thiophene transformed per gram of catalyst and per second) of the catalyst with a SiC support is substantially lower than that of the catalyst with an alumina support by a factor of close to 2 to 6 depending on the surface areas of the SiC support used and close to that of a catalyst based on alumina.

The technical teaching of that patent is that the SiC support can overcome problems connected with the presence of high molecular weight aromatics or metals in the feed. That patent, in contrast, does not indicate the advantage of using a SiC support in place of alumina with respect to hydrodesulphurization activity. Further, it neither mentions nor suggests any advantages in using such a catalyst for selective hydrodesulphurization of olefinic feeds in accordance with the invention (feeds substantially free of the undesirable compounds already mentioned).

In contrast, the Applicant has surprisingly discovered that for the selective hydrodesulphurization of olefinic feeds, the use of catalysts with a support essentially constituted by β has substantial advantages over known catalysts. It appears that the use of SiC is advantageous as it can produce a catalyst with improved selectivity, and can minimize the start up time for said catalyst by limiting the initial hydrogenating overactivity. Such a problem has been reported for commercial catalysts supported on alumina, and U.S. Pat. No. 4,149,965 teaches prior deactivation of the catalyst prior to its use in hydrotreating an olefinic feed, the deactivation treatment being selected so as to limit the hydrogenating activity of the catalyst.

Without wishing to be bound to a particular theory, this effect could be considered to derive from the properties of the support, which could be both substantially non acidic and substantially non basic. It is known that the hydrogenating properties of catalysts are enhanced by the acidity of the support. The absence of acidity could contribute to explaining the results obtained, thanks to a relatively low hydrogenating activity of the catalyst with a β SiC support as regards olefins. This could encourage the hydrodesulphurization selectivity as regards olefin hydrogenation. It cannot be excluded that the nature of the sulphide phase and thus its catalytic properties could be different, depending on whether it is formed on an alumina support or on a β silicon carbide support.

The β silicon carbide used in the invention typically has a specific surface area (measured by the BET technique) that is over 5 m²/g, preferably in the range 5 to 300 m²/g, and more preferably in the range 10 to 250 m²/g. The pore volume of the support is typically in the range 0.20 cm³/g to 1.0 cm³/g, preferably in the range 0.3 cm³/g to 0.8 cm³/g, and more preferably in the range 0.35 cm³/g to 0.65 cm³/g.

The amount of group VI metal, in moles per gram of support, is typically in the range 6.94×10⁻⁵ to 1.40×10⁻³, preferably in the range 8.34×10⁻⁵ to 6.95×10⁻⁴ and highly preferably in the range 1.04×10⁻⁴ to 5.50×10⁻⁴. The amount of group VIII metal, in moles per gram of support, is generally in the range 4.0×10⁻⁵ to 1.1×10⁻³, preferably in the range 5.34×10⁻⁵ to 5.34×10⁻⁴, and highly preferably in the range 6.0×10⁻⁵ to 4.0×10⁻⁴. The catalyst can also advantageously contain phosphorus the content of which, in moles per gram of support, can be in the range 1.64×10⁻⁵ to 1.64×10⁻³, preferably in the range 8.2×10⁻⁵ to 1.31×10⁻³, and highly preferably in the range 8.2×10⁻⁵ to 6.6×10⁻⁴.

The manufacture of silicon carbide type supports which can be used in heterogeneous catalysis is already known and disclosed in patents such as EP-B1-0 440 569 or in U.S. Pat. No. B1-6,184,178, although this list is not exhaustive. That type of support is manufactured on an industrial scale, for example by SICAT Sarl (France).

The catalysts of the invention can be prepared using any standard preparation method that is known to the skilled person, the different metals of the active phase also possibly being deposited sequentially or simultaneously on the support. Non exhaustive examples which can be cited are dry impregnation preparation methods, exchange methods, or surface organometallic chemical methods.

The catalysts can be presulphurized in situ or ex situ prior to use in the selective hydrodesulphurization process. Sulphurization can be carried out using any method that is known to the skilled person. As an example, the catalyst can be placed in an atmosphere of H₂S diluted with a stream of hydrogen at a predetermined temperature for a predetermined period.

In general, the invention concerns the use of supported catalysts comprising at least one metal or metallic compound of a metal from the group formed by elements from groups VIII and/or VIB of the periodic table, deposited on a support essentially constituted by β silicon carbide, in a process for selective hydrodesulphurization of an olefinic hydrocarbon feed substantially free of polynuclear aromatics and metals.

Typically, the feed has an end point of less than 260° C., and comprises at least 5% by weight of olefms. Generally, said feed boils in the gasoline range, i.e. in the ASTM boiling point range of about 30° C. to 230° C. As an example, the feed comprises at least 50% by weight of pyrolysis gasoline and/or fluid catalytic cracking gasoline, and may even be constituted by more than 90% by weight, or even entirely constituted by gasoline fractions or gasoline deriving from a steam cracker and/or a fluid catalytic cracker (FCC).

Usually, the selective hydrodesulphurization process is carried out under temperature conditions in the range 200° C. to 400° C., at a pressure in the range 0.5 to 4.0 MPa, with a H₂/HC ratio in the range 100 to 600 (litre/litre under normal conditions) and with an hourly mass flow rate of feed per unit weight of catalyst (WHSV) in the range 1 to 15 h⁻¹.

EXAMPLES Example 1 Preparation of a Catalyst A for Use in Accordance with the Invention

A catalyst A was obtained by using a synthesis method termed the OrganoMetallic Surface Chemical method (OMSC). Silicon carbide SiC extrudates (2 mm diameter) were supplied by SICAT Sarl; their principal characteristics are summarized in Table 1. TABLE 1 Characteristics of SiC support Form Surface area: S_(BET) m²/g Pore volume (Hg) cm³/g Extrudates 53 0.4 2 mm

An aqueous solution of ammonium heptamolybdate was impregnated using the pore volume method into the silicon carbide. The molybdenum (Mo) concentration in the solution was calculated to obtain the desired Mo content on the support, then the solid was left to mature for 12 hours. The solid was then oven dried at 120° C. for twelve hours, and calcined for two hours at 500° C. in a stream of dry air (1 l/h.g of catalyst). The solid was then sulphurized in a stream of gaseous H₂S in hydrogen (15% by weight of H₂S, total gas flow rate 1 l/h.g of catalyst) from ambient temperature to 400° C. (5° C./min ramp-up). The temperature was kept at 400° C. for two hours, then the system was cooled to 200° C. (ramp-down 5° C./min) and maintained at that temperature for an additional two hours in pure hydrogen, then finally cooled to ambient temperature, still in pure hydrogen. The pretreated solid was transferred into a reactor suitable for OMSC synthesis (Schlenk tube). This reactor had already been filled with solution so that the volume of the solution was 10 cm³/g of catalyst, then purged with argon to eliminate all traces of oxygen from the medium. The solution was constituted by an organometallic Co complex, cobalt biscyclopentadienyl Co(C₅H₅)₂ diluted in n-heptane. The concentration of organometallic complex was selected to obtain a Co/(Co+Mo) atomic ratio of 0.4. The solid was left in solution for two hours at ambient temperature and hydrogen bubbled through, then washed in pure heptane and dried in a stream of argon at ambient temperature overnight. Finally, the solid was sulphurized by applying the same treatment as above. The characteristics of the catalyst following sulphurization are shown in Table 2. TABLE 2 Characteristics of catalyst A (in accordance with use in the invention) Mo content Co content Co/(Co + Mo) atomic ratio (wt %) (wt %) (atom/atom) 3.1 0.7 0.37

Example 2 Preparation of a Catalyst B (Comparative)

Catalyst B was obtained using the same synthesis protocol as for catalyst A, with an industrial alumina type support from Axens. The characteristics of the support are given in Table 3: TABLE 3 Characteristics of industrial alumina support Form Surface area: S_(BET) m²/g Pore volume (Hg) cm³/g Beads 60 0.6 2.4-4 mm

The characteristics of the catalyst after sulphurization are shown in Table 4: TABLE 4 Characteristics of catalyst B (comparative) Mo content Co content Co/(Co + Mo) atomic ratio (wt %) (wt %) (atom/atom) 3.2 0.7 0.36

Thus, catalyst B is essentially distinguished from catalyst A in the nature of the support used, and also by the dimensions of the grains.

Example 3 Comparison of Use of Catalyst A with that of Catalyst B on a First Olefinic Feed

In order to overcome diffusional limitation problems, the catalysts were ground to the 300-500 micrometre fraction in the absence of air. The solids were then passivated in air at ambient temperature for 4 hours and loaded into the catalytic reactor. The catalyst was then sulphurized in situ using a synthetic feed (6% by weight of dimethyldisulphide in n-heptane) under the following conditions: Total pressure=2.0 MPa, H₂/feed=300 (litre/litre), mass flow rate of feed with respect to catalyst per hour (WHSV)=3 h⁻¹. A constant temperature stage for sulphurization was carried out for 4 hours at 350° C. (temperature ramp-up 20° C./hour). After sulphurization, the temperature was reduced to 150° C. and the sulphurization feed was replaced with FCC gasoline to be treated, and the operating conditions were adjusted. In this example, the two catalysts were tested on a first olefinic feed constituted by a moderately sulphurized total FCC gasoline with the characteristics shown in Table 5. TABLE 5 Characteristics of first olefinic feed Total S: 460 ppm by weight Density (25° C.): 0.76 PONA analysis (wt %): Paraffins: 28.4 Naphthenes; 8.1 Aromatics: 29.3 Olefins: 34.2

The test conditions were as follows: total pressure: 1.5 MPa;

H₂/feed=300 (litre/litre), mass flow rate of feed with respect to catalyst per hour (WHSV)=9 h⁻¹.

The temperature was varied between 280° C. and 310° C. Each operating condition (temperature) was kept constant for at least 48 hours, and a reversal point ensured that the loss of desulphurizing activity of the catalyst was very small or even zero. The degrees of HDS and HDO are calculated respectively using the following formulae: HDS=100×(1−S _(f) /S _(o)), in which S_(o) and S_(f) respectively represent the concentrations in the feed and effluent (ppm); HDO=100×(1−C _(f) /C _(o)) in which C_(o) and C_(f) represent the concentrations of olefins in the feed and in the effluent respectively (wt %).

The hydrodesulphurization and hydrogenation activities were calculated by assuming an order of 1 (sulphur-containing compounds ) and 0 (olefinic compounds) respectively for the reactants: A _(HDS) =Ln(100(100−HDS)); A _(HDO) =C _(c) ×HDO/100.

Table 6 shows the results obtained for catalysts A and B. It appears that the catalyst supported on silicon carbide was much more selective than the catalyst supported on alumina since catalyst A was systematically less hydrogenating than catalyst B. TABLE 6 selective hydrodesulphurization of a first olefinic feed Test HDS HDO Catalyst T (° C.) duration (h) (%) (%) A_(HDA) A_(HDO) A 280 96 74.5 12.1 1.37 0.040 B 280 96 73.3 15.2 1.31 0.051 A 290 144 86.8 19.2 2.02 0.063 B 290 144 86.2 24.3 1.97 0.081 A 300 192 94.0 29.4 2.81 0.097 B 300 192 93.2 33.3 2.66 0.111

More precisely, the use of a silicon carbide support in place of alumina can reduce the hydrogenating activity of the catalyst while its desulphurizing activity remains constant or may even be slightly improved.

Example 4 Start Up Operation of Catalyst A and Catalyst B in Selective Hydrodesulphurization of a First Olefinic Feed

The changes in hydrodesulphurization and hydrogenation during the first 96 hours of the test of Example 3 for catalyst A and catalyst B are shown in Table 7 and in FIG. 1. TABLE 7 Change in degree of hydrodesulphurization and hydrogenation during start up of catalysts A and B during selective hydrodesulphurization of a first olefinic feed HDS HDO Catalyst Test duration (h) (%) (%) A 10 75.0 12.8 B 12 74.5 20.1 A 24 74.8 12.5 B 24 74.5 18.2 A 36 74.6 12.2 B 38 73.9 17.1 A 48 74.5 12.1 B 48 73.5 16.3 A 72 74.2 12.3 B 72 73.2 15.4 A 96 74.5 12.1 B 96 73.3 15.2

During the first hours of the test, catalyst B (not in accordance with the catalyst used in the invention) exhibited hydrogenating overactivity compared with its stabilized state, while its desulphurizing activity was essentially stable. Then, to achieve a similar sulphur content, catalyst B (not in accordance) resulted in hydrogenation of a surplus of olefins in the feed at the start of the test, and the quality of the gasoline obtained at the start of the cycle on catalyst B was thus lower in terms of octane number than that obtained for the same catalyst after stabilization. For this type of catalyst, a partial deactivation treatment such as that proposed in U.S. Pat. No. 4,149,965 is thus recommended to limit its hydrogenating activity prior to passing the feed. In contrast, this phenomenon is substantially reduced on catalyst A (in accordance with the use of the invention), which had no substantial hydrogenating overactivity at the start of the test and very rapidly reached its steady state. Thus it was not necessary to partially deactivate catalyst A prior to bringing it into contact with the feed.

Example 5 Comparison of Catalyst A with Conventional Catalyst B on a Second Olefinic Feed

In this example, the two above catalysts were tested on a depentanized olefinic feed (Table 8) more sulphurized as above. TABLE 8 Characteristics of second olefinic feed Total S: 2297 ppm by weight Density (25° C.): 0.77 PONA analysis (wt %): Paraffins: 24.5 Naphthenes; 8.4 Aromatics: 37.0 Olefins: 30.1

The test conditions were as follows: total pressure: 1.8 MPa; H₂/feed=350 (litre/litre), WHSV=7 h⁻¹. The temperature was varied between 280° C. and 310° C. to vary the degree of desulphurization. Table 9 shows the results obtained for catalysts A and B: TABLE 9 Selective hydrodesulphurization of a second olefinic feed Test HDS HDO Catalyst T (° C.) duration (h) (%) (%) A_(HDA) A_(HDO) A 280 96 77.9 13.7 1.51 0.046 B 280 96 77.1 16.7 1.47 0.056 A 290 144 87.3 18.0 2.06 0.060 B 290 144 86.7 20.5 2.02 0.068 A 300 192 92.9 22.2 2.65 0.074 B 300 192 92.5 25.0 2.59 0.083 A 310 240 95.8 26.5 3.17 0.088 B 310 240 95.6 30.1 3.12 0.100

For this feed again, the catalyst supported on silicon carbide proved to be more selective. The best selectivity for catalyst A was again due to a lower hydrogenating activity for catalyst A, while the desulphurization activity of the two catalysts was substantially identical.

Without further elaboration, it is believed that one skilled in the art can, using the preceding description, utilize the present invention to its fullest extent. The preceding preferred specific embodiments are, therefore, to be construed as merely illustrative, and not limitative of the remainder of the disclosure in any way whatsoever.

In the foregoing and in the examples, all temperatures are set forth uncorrected in degrees Celsius and, all parts and percentages are by weight, unless otherwise indicated.

The entire disclosure[s] of all applications, patents and publications, cited herein and of corresponding French application No. 03/10.027, filed Aug. 19, 2003 is incorporated by reference herein.

The preceding examples can be repeated with similar success by substituting the generically or specifically described reactants and/or operating conditions of this invention for those used in the preceding examples.

From the foregoing description, one skilled in the art can easily ascertain the essential characteristics of this invention and, without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions. 

1. Use of supported catalysts comprising at least one metal or metallic compound of a metal from the group formed by elements from group VIII and/or group VIB of the elements periodic table deposited on a support essentially constituted by β silicon carbide, in a process for selective hydrodesulphurization of an olefinic hydrocarbon feed that is substantially free of polynuclear aromatics and metals.
 2. Use according to claim 1, in a process for selective hydrodesulphurization of a feed with an end point of less than 260° C., comprising at least 5% by weight of olefins.
 3. Use according to claim 2, in a process for selective hydrodesulphurization of a feed boiling in the gasoline range.
 4. Use according to claim 3, in a process for selective hydrodesulphurization of a feed comprising at least 50% by weight of pyrolysis gasoline and/or fluid catalytic cracking gasoline.
 5. Use according to claim 1, in which the support has a specific surface area in the range 5 to 300 m²/g.
 6. Use according to claim 1 in a process for selective hydrodesulphurization carried out under temperature conditions in the range 200° C. to 400° C., a pressure in the range 0.5 to 4 MPa, with a H₂/feed ratio in the range 100 to 600 (litre/litre) and with an hourly mass flow rate of feed with respect to catalyst (WHSV) in the range 1 to 15 h⁻¹. 